OP495 Analog Devices, OP495 Datasheet - Page 11

• Manufacturer Part Number: OP495
• Description: Quad Rail-to-Rail Operational Amplifier
• Manufacturer: Analog Devices

Specifications of OP495

• -3db Bandwidth: 85kHz
• Slew Rate: 30mV/µs
• Vos: 300µV
• Ib: 20nA
• # Opamps Per Pkg: 4
• Input Noise (nv/rthz): 45nV/rtHz
• Vcc-vee: 3V to 36V
• Isy Per Amplifier: 175µA
• Packages: DIP,SOIC

Resistor R
Minimum gain is 6 (with no R
in absolute value as well as temperature coefficient to maximize
common-mode rejection performance and minimize drift. This
instrumentation amplifier can operate from a supply voltage as
low as 3 V.
SINGLE-SUPPLY RTD THERMOMETER AMPLIFIER
This RTD amplifier takes advantage of the rail-to-rail swing of
the OP295/OP495 to achieve a high bridge voltage in spite of a
low 5 V supply. The OP295/OP495 amplifier servos a constant
200 μA current to the bridge. The return current drops across
the parallel resistors 6.19 kΩ and 2.55 MΩ, developing a voltage
that is servoed to 1.235 V, which is established by the
band gap reference. The 3-wire RTD provides an equal line
resistance drop in both 100 Ω legs of the bridge, thus improving
the accuracy.
The AMP04 amplifies the differential bridge signal and converts
it to a single-ended output. The gain is set by the series resis-
tance of the 332 Ω resistor plus the 50 Ω potentiometer. The
gain scales the output to produce a 4.5 V full scale. The 0.22 μF
capacitor to the output provides a 7 Hz low-pass filter to keep
noise at a minimum.
2.55MΩ
+
V
–
REF
V
1%
IN
10-TURNS
26.7kΩ
100Ω
RTD
100kΩ
G
0.5%
Figure 24. Single-Supply Instrumentation Amplifier
R1
sets the gain of the instrumentation amplifier.
200Ω
6.19kΩ
1%
100Ω
0.5%
Figure 25. Low Power RTD Amplifier
3
2
ZERO ADJ
V
O
+
–
=
20kΩ
26.7kΩ
0.5%
AD589
(
R2
5 + 200kΩ
OP295/
OP495
2
1/2
–
1
R
1
R
G
+
3
G
G
). All resistors should be matched
OP295/
20kΩ
)
1.235
OP495
R3
V
IN
1/2
37.4kΩ
+ V
REF
3
2
5
6
AMP04
5V
7
+
–
+
–
4
100kΩ
1
V+
8
4
R4
5V
5
8
OP295/
OP495
7
0.22µF
50Ω
332Ω
1/2
6
4.5V = 450°C
0V = 0°C
AD589
V
O
V
O
Rev. G | Page 11 of 16
COLD JUNCTION COMPENSATED, BATTERY-
POWERED THERMOCOUPLE AMPLIFIER
The 150 μA quiescent current per amplifier consumption of the
OP295/OP495 makes them useful for battery-powered temperature
measuring instruments. The K-type thermocouple terminates
into an isothermal block where the terminated junctions’ ambient
temperatures can be continuously monitored and corrected by
summing an equal but opposite thermal EMF to the amplifier,
thereby canceling the error introduced by the cold junctions.
K-TYPE
THERMOCOUPLE
40.7µV/°C
To calibrate, immerse the thermocouple measuring junction in
a 0°C ice bath and adjust the 500 Ω zero-adjust potentiometer
to 0 V out. Then immerse the thermocouple in a 250°C tem-
perature bath or oven and adjust the scale-adjust potentiometer
for an output voltage of 2.50 V, which is equivalent to 250°C.
Within this temperature range, the K-type thermocouple is
quite accurate and produces a fairly linear transfer characteristic.
Accuracy of ±3°C is achievable without linearization.
Even if the battery voltage is allowed to decay to as low as 7 V,
the rail-to-rail swing allows temperature measurements to 700°C.
However, linearization may be necessary for temperatures above
250°C, where the thermocouple becomes rather nonlinear. The
circuit draws just under 500 μA supply current from a 9 V
battery.
5 V ONLY, 12-BIT DAC THAT SWINGS 0 V TO 4.095 V
Figure 27 shows a complete voltage output DAC with wide
output voltage swing operating off a single 5 V supply. The
serial input, 12-bit DAC is configured as a voltage output device
with the 1.235 V reference feeding the current output pin (I
of the DAC. The V
the output.
The output voltage from the DAC is the binary weighted voltage
of the reference, which is gained up by the output amplifier such
that the DAC has a 1 mV per bit transfer function.
ALUMEL
CHROMEL
+
–
AL
CR
Figure 26. Battery-Powered, Cold-Junction Compensated
ISOTHERMAL
BLOCK
1N914
COLD
JUNCTIONS
1.5MΩ
AD589
REF
1%
, which is normally the input, now becomes
Thermocouple Amplifier
7.15kΩ
24.9kΩ
475Ω
1%
1%
1%
1.235V
4.99kΩ
500Ω
10-TURN
24.3kΩ
1%
2.1kΩ
1%
ZERO
ADJUST
1%
24.9kΩ
9V
2
3
OP295/OP495
–
+
+
–
8
4
1.33MΩ
OP295/
OP495
ADJUST
SCALE
1
20kΩ
5V = 500°C
0V = 0°C
OUT
V
O
)